This application is based on application No. 11-276036 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to image processing apparatuses and particularly to an image processing apparatus that corrects chromatic aberration caused by an optical system in digital processing of image information obtained through optical reading of an original document image.
2. Description of the Related Art
Regarding conventional digital copying machines and the like, color deviation (chromatic aberration) of an optical system occurs on edges in the main scanning direction, due to chromatic aberration of a lens employed in a reading unit. FIG. 10 illustrates the principle of chromatic aberration of a lens. It is supposed here that black lines are printed on an original document 10 at respective positions x1, x2 and x3 in the main scanning direction.
Referring to FIG. 10, light rays reflected from the black line at the position x2 are passed straight through the center of a lens and then focused on a CCD (charge-coupled device) and accordingly the reflected light rays are focused at the same position. In other words, chromatic aberration of the lens does not affect the reflected light rays. The CCD can then read the reflected light rays as black data including three colors of R (red), G (green) and B (blue) having the same barycenters and peak values.
On the other hand, light rays reflected from the black line at the position x1 or x3 are directed from the edge region of the lens and focused on the CCD, and therefore affected by the chromatic aberration of the lens. Specifically, light rays of RGB incident on the lens are refracted according to respective wavelengths and focused at different positions so that respective barycenters and peak values of these three colors differ from each other. In particular, as shown in the lower part of FIG. 10, long-wavelength light (R) condenses inward on the CCD while short-wavelength light (B) condenses outward on the CCD.
FIG. 11 shows chroma relative to addresses in the direction of main scanning for explaining the above chromatic aberration. As seen from FIG. 11, the chroma relative to edge parts and therearound such as x1 and x3 in the main scanning direction is higher because of the influence of chromatic aberration while the chroma relative to the central part like x2 which is not affected by the chromatic aberration is lower as compared therewith.
The chromatic aberration as discussed above causes no problem for an image having a relatively even density distribution such as a color patch. However, for an image where density abruptly changes such as a character image, the chromatic aberration generates color deviation on the edge portion thereof. Especially, on the edge of a black character, an erroneous judgement due to the chromatic aberration results in blurring of color around the character, partial loss of the character, and the like.
A lens of high quality is thus required in a PPC (plain paper electric copying machine) employing a color CCD, however, such a lens cannot satisfactorily meet requirements. Specifically, improvement of lens performance is accompanied by increase in size of a lens system, resulting in increase in size of an entire machine including an optical system of a scanner. Further, there is a considerable difference in quality and performance between lens parts. A method is then necessary for correcting the chromatic aberration finally by an image processing system.
A method is now described of correcting chromatic aberration that has been employed in an image processing system of a conventional art. The chromatic aberration is generally corrected by mixing data on pixels adjoining each other using chromatic aberration correction coefficients as represented by the following equations:
R(n)=a1(n)xc3x97R(nxe2x88x921)+a2(n)xc3x97R(n)+a3(n)xc3x97R(n+1),
G(n)=G(n),
B(n)=a3(n)xc3x97B(nxe2x88x921)+a2(n)xc3x97B(n)+a1(n)xc3x97B(n+1),
where n represents the position of a target pixel relative to a reference position of the main scanning, and a1(n), a2(n) and a3(n) represent correction coefficients for the target pixel which is the nth pixel.
FIG. 12 is a block diagram illustrating a general method of correcting chromatic aberration according to the conventional art. Referring to FIG. 12, in order to correct chromatic aberration according to the conventional art, data on RGB read by a reading unit 101 is corrected by a chromatic aberration correcting unit 109 using correction coefficients (a1, a2, a3) calculated by a correction coefficient calculating unit 103.
If predetermined values are used respectively as the correction coefficients (a1, a2, a3), correction would be accomplished for a state different from the actual state of chromatic aberration since manufactured lenses have different qualities and performances. It is thus necessary to determine the actual state of chromatic aberration for each machine and then determine correction coefficients a1(n), a2(n) and a3(n) which are appropriate for each machine. However, measurement of the chromatic aberration for each machine is inefficient in terms of production efficiency and the like. Accordingly, a method is actually employed as explained below.
FIG. 13 illustrates pseudo-calculation of correction coefficients by correction coefficient calculating unit 103 in FIG. 12. Referring to FIG. 13, RGB data read by reading unit 101 is transmitted first to chromatic aberration correcting units 1301, 1302 and 1303 in respective blocks where three sets of chromatic aberration correction coefficients ([a11, a12, a13], [a21, a22, a23], [a31, a32, a33]) calculated at the time of lens design are used to correct chromatic aberration for each pixel ([R1, G1, B1], [R2, G2, B2], [R3, G3, B3]).
Chroma data (MAX(R,G,B)xe2x88x92MIN(R,G,B)) is then calculated for each block (M1, M2, M3) by a chroma calculating unit 1305 and thereafter the minimum one (Y) of them is determined by a MIN unit 1307. RGB data ([RY, GY, BY]) is finally determined by a selector 1309 that is data obtained by correction of chromatic aberration associated with the minimum value Y.
In this way, RGB data of any block that allows the chroma data to be minimum for respective colors is selected to calculate chromatic aberration correction coefficients in a pseudo-manner for correcting chromatic aberration. In other words, instead of using fixed chromatic aberration correction coefficients calculated for each machine, optimum chromatic aberration correction coefficients are selected from three sets of chromatic aberration correction coefficients which are calculated in advance. Then, the optimum chromatic aberration correction coefficients are used to accomplish correction of chromatic aberration.
Since the influence of chromatic aberration is noticeable on the edge part of a black character and is inconspicuous on the remaining part thereof, such a chromatic aberration correction has been considered to be satisfactory in the practical use. Accordingly, this method has been regarded as the one which saves labor of measuring chromatic aberration for each machine and thus achieves an easier and more appropriate correction of chromatic aberration.
However, according to this conventional art, an image processing apparatus always selects, from predetermined chromatic aberration correction coefficients, those correction coefficients which provide the minimum chroma. Therefore, depending on image data, the selected correction coefficients may be different from those suitable for the actual chromatic aberration. Consequently, a problem occurs that a thin line of a single color for example RGB cannot be reproduced.
FIG. 14 shows an image of an original document having a ladder pattern with one line per n-dot. FIG. 15 shows density values obtained by reading pixels adjoining each other in the right edge region of the ladder pattern in FIG. 14. As discussed above, R on the edge of the original document image deviates inward while B deviates outward. Consequently, when the ladder pattern as shown in FIG. 14 is read, respective barycenters of RGB deviate from each other on the edge of the black line in the edge region.
R and G thus have respective density values different from each other as shown in FIG. 15. It is noted that B is not shown here for allowing the difference to be distinguished easily. Respective density values of R and G should inherently be equal to each other when a black line is read. However, R has a density value of 40 higher than that of 20 of G when a pixel g1 is read while R has a density value of 60 lower than that of 80 of G when an adjacent pixel g2 is read due to chromatic aberration.
When such density values are obtained, the extent of deviation of colors can easily be calculated based on data values of a sample to be read if the sample is originally known. Actually, however, any sample to be read is unknown at the beginning and it is thus impossible to judge whether the deviation of barycenters as shown is caused by chromatic aberration or it is normal data.
According to the conventional art, chromatic aberration is corrected to achieve the minimum chroma all the time. Therefore, in terms of each pixel, selected chromatic aberration correction coefficients for a pixel may produce an effect which is entirely different from that produced by chromatic aberration correction coefficients for a pixel adjacent to that pixel, for example.
Actually, it seems unlikely that a state of chromatic aberration for one pixel is entirely different from that for an adjacent pixel. If chromatic aberration correction coefficients producing entirely different effects are selected, chromatic aberration, which actually occurs, is improperly corrected. For this reason, a thin line of a single color such as a thin green line or a thin red line could be corrected to a thin black line having a low chroma and thus cannot be reproduced appropriately.
In view of this, Japanese Patent Laying-Open No. 11-69105 discloses a technique of correcting color deviation suitable for an actual state of chromatic aberration. According to a disclosed method, chromatic aberration correction coefficients are actually calculated instead of selection of optimum coefficients from predetermined sets of chromatic aberration correction coefficients.
Specifically, according to this method, appropriate chromatic aberration correction coefficients are calculated based on spatial frequency components of an original image to be read.
However, the technique disclosed in Japanese Patent Laying-Open No. 11-69105 is difficult to implement as hardware. Even if it is possible, the resultant circuit is great in size, raising the cost.
One object of the present invention is accordingly to provide an image processing apparatus capable of easily correcting chromatic aberration according to an actual state of chromatic aberration by correcting chromatic aberration correction coefficients.
This object is accomplished by an image processing apparatus including following components. Specifically, according to one aspect of the invention, the image processing apparatus includes an image reading unit, a calculating unit calculating a chromatic aberration correction coefficient by using image data read by the image reading unit, a first correcting unit correcting the calculated chromatic aberration correction coefficient, and a second correcting unit correcting image data of an original read by the image reading unit by using the corrected chromatic aberration correction coefficient.
According to the present invention, the calculated chromatic aberration correction coefficient is corrected and thus it is possible to provide an image processing apparatus capable of easily correcting chromatic aberration according to an actual state of chromatic aberration.
Preferably, the image processing apparatus further includes a storing unit storing the calculated chromatic aberration correction coefficient. The first correcting unit includes a first reading unit reading the stored chromatic aberration correction coefficient from the storing unit, and a second reading unit reading from the storing unit a chromatic aberration correction coefficient adjoining the read chromatic aberration correction coefficient. The chromatic aberration correction coefficient read by the first reading unit is corrected by using the adjoining chromatic aberration correction coefficient read by the second reading unit.
In this way, the chromatic aberration correction coefficient is corrected by using the adjoining chromatic aberration correction coefficient. It is then avoided to inconveniently select improper chromatic aberration correction coefficients respectively for a target pixel and a pixel adjacent thereto.
Preferably, the first correcting unit takes an average of the chromatic aberration correction coefficient read by the first reading unit and the adjoining chromatic aberration correction coefficient read by the second reading unit to correct the chromatic aberration correction coefficient read by the first reading unit.
In this way, the chromatic aberration correction coefficient can be corrected in a relatively easy manner because of a light load of a circuit for calculating the average.
Preferably, the first correcting unit takes a median of the chromatic aberration correction coefficient read by the first reading unit and the adjoining chromatic aberration correction coefficient read by the second reading unit to correct the chromatic aberration correction coefficient read by the first reading unit.
In this way, the median is selected in a certain region and accordingly any spike noise occurring in the calculated chromatic aberration correction coefficient is eliminated.
Preferably, the first correcting unit performs expansion and erosion processing on the chromatic aberration correction coefficient read by the first reading unit by using the adjoining chromatic aberration correction coefficient read by the second reading unit so as to correct the chromatic aberration correction coefficient read by the first reading unit.
In this way, the expansion and erosion processing is carried out to allow any noise produced in the calculated chromatic aberration correction coefficient to be eliminated.
Preferably, the calculating unit performs an arithmetic operation for correcting chromatic aberration by using a plurality of chromatic aberration correction coefficients and selects a chromatic aberration correction coefficient which allows chroma data obtained by the arithmetic operation to be minimum.
In this way, the chromatic aberration correction coefficient allowing chroma data to be minimum is selected from the multiple number of chromatic aberration correction coefficients. The chromatic aberration correction coefficient before corrected can easily and appropriately be determined.
Preferably, the second correcting unit multiplies the image data by the corrected chromatic aberration correction coefficient and performs addition on resultant products to correct the image data.
In this way, image data is properly corrected by using the corrected chromatic aberration correction coefficient.
According to another aspect of the invention, an image processing apparatus includes an image reading unit, a calculating unit calculating a chromatic aberration correction coefficient for each pixel in a main scanning direction by using image data read by the image reading unit, a first correcting unit correcting a chromatic aberration correction coefficient for a target pixel by using a chromatic aberration correction coefficient for a pixel adjoining the target pixel, and a second correcting unit correcting image data of an original read by the image reading unit by using the corrected chromatic aberration correction coefficient.
According to the present invention, a chromatic aberration correction coefficient calculated for each pixel in the main scanning direction is corrected by using a chromatic aberration correction coefficient for an adjoining pixel. It is thus avoided to inconveniently use improper chromatic aberration correction coefficients for the target pixel and the adjoining pixel in the main scanning direction respectively.
Preferably, the first correcting unit performs matrixing operation based on an average filter to correct the chromatic aberration correction coefficient.
In this way, the chromatic aberration correction coefficient can be corrected in a relatively easy manner because of a light load of a circuit for performing matrixing based on the average filter.
Preferably, the first correcting unit performs matrixing operation based on a median filter to correct the chromatic aberration correction coefficient.
In this way, a median is selected in a certain region in a matrix according to the matrixing based on the median filter, and accordingly any spike noise occurring in the calculated chromatic aberration correction coefficient can be prevented.
Preferably, the first correcting unit performs matrixing operation based on a morphology filter to correct the chromatic aberration correction coefficient.
In this way, any noise produced in the calculated chromatic aberration correction coefficient can be eliminated by the matrixing based on the morphology filter.
Preferably, the calculating unit performs arithmetic operation for correcting chromatic aberration of image data of each pixel by using a plurality of chromatic aberration correction coefficients and selects a chromatic aberration correction coefficient which allows chroma data obtained by the arithmetic operation to be minimum.
In this way, the chromatic aberration correction coefficient before corrected can easily and properly be determined by selecting from the multiple number of chromatic aberration correction coefficients the chromatic aberration correction coefficient allowing the chroma data to be minimum.
Preferably, the second correcting unit multiplies the image data by the corrected chromatic aberration correction coefficient and performs addition on resultant products to correct the image data.
In this way, image data is properly corrected by using the corrected chromatic aberration correction coefficient.
According to a further aspect of the invention, an image processing method for correcting chromatic aberration of image data read by an image reading unit includes the steps of calculating a chromatic aberration correction coefficient by using the image data read by the image reading unit, correcting the calculated chromatic aberration correction coefficient, and correcting by using the corrected chromatic aberration correction coefficient image data of an original read by the image reading unit.
According to the present invention, the calculated chromatic aberration correction coefficient is corrected and thus it is possible to provide an image processing method which enables an easy correction of chromatic aberration according to an actual state of chromatic aberration.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.